Coextruded pressure sensitive label stock material with integral peelable backing

ABSTRACT

An opaque, biaxially oriented label stock product of lustrous appearance is provided which comprises: 
     (a) a facing layer which is a laminate of 
     (i) a thermoplastic polymer matrix substrate layer having an upper surface and a lower surface, a pressure sensitive adhesive component incorporated within said substrate layer or applied as a coating to the lower surface thereof and a strata of voids located within said substrate layer; 
     positioned at least substantially within at least a substantial number of said voids is at least one spherical void-initiating particle which is phase distinct and incompatible with the matrix material, the void space occupied by said particle being substantially less than the volume of said void, with one generally cross-sectional dimension of said particle at least approximating a corresponding cross-sectional dimension of said void; the population of voids in said substrate and the thickness of said substrate being such as to cause a degree of opacity of less than 70 percent light transmission; and 
     (ii) a void-free thermoplastic skin layer adhering to the upper surface of said substrate layer, said skin layer being of a thickness such that the outer surface thereof substantially conceals the surface irregularities of said substrate layer; and, 
     (b) a release layer in the form of a thermoplastic film having an upper surface and a lower surface and having a release agent component for the pressure sensitive adhesive component of label layer (a)(i) incorporated therein or applied as a coating to the upper surface thereof, the lower surface of substrate (a)(i) being peelably affixed to the upper surface of said release layer (b).

BACKGROUND OF THE INVENTION

This invention relates to the field of pressure sensitive adhesivelabels.

In the conventional manufacture of pressure sensitive adhesive labels, asolution of a pressure sensitive material is applied to a substrate, orface stock, usually paper, in the form of a continuous web and dried.Thereafter a release sheet, usually provided as a continuous paper webcoated with a release agent, is applied to the exposed pressuresensitive adhesive surface to form a substrate-pressure sensitiveadhesive-release sheet label stock which is then wound on a rewind rollfor further processing such as sheeting, slitting or other converting.After that, the label stock is typically printed, cut and collated by aprinter to form individual printed sheets. Such sheets may be utilizedfor any number of uses including name tags, stickers, labels, etc., bysimply peeling away the release paper and pressing the adhesive-coatedside of the face sheet to the desired surface.

U.S. Pat. No. 4,377,616, the contents of which are incorporated byreference herein, discloses an opaque biaxially oriented polymer filmstructure comprising a thermoplastic polymer matrix core layerpossessing numerous voids, a substantial number of which contain atleast one spherical void-initiating particle, and transparent skinlayers adhering to the surfaces of the core layer. The unique structureof the core layer imparts a much higher degree of opacity, possibly dueto the effects of light scattering, than that possible by the use ofopacifying pigment alone.

There is no suggestion, however, in U.S. Pat. No. 4,377,616 ofincorporating the film structure disclosed therein in a pressuresensitive label stock.

SUMMARY OF THE INVENTION

It is an object of the present invention to manufacture a pressuresensitive label stock material entirely from synthetic resin componentsin a single coextrusion operation employing conventional equipment.

It is a further object of the invention to provide a biaxially orientedlabel stock product possessing a facing layer having a pressuresensitive adhesive material incorporated therein or applied to thesurface thereof coextruded with, and peelably affixed to, a releaselayer having a release material incorporated therein or applied to thesurface thereof adjacent the facing layer.

It is still another object of the invention to provide a biaxiallyoriented label stock product in which the facing layer features anopaque expanded thermoplastic resin core, or substrate, of lustrousappearance as in aforesaid U.S. Pat. No. 4,377,616.

In keeping with the foregoing objects, there is provided an opaque,biaxially oriented label stock product of lustrous appearance whichcomprises:

(a) a facing layer which is a laminate of

(i) a thermoplastic polymer matrix substrate layer having an uppersurface and a lower surface, a pressure sensitive adhesive componentincorporated within said substrate layer or applied as a coating to thelower surface thereof and a strata of voids located within saidsubstrate layer;

positioned at least substantially within at least a substantial numberof said voids is at least one spherical void-initiating particle whichis phase distinct and incompatible with the matrix material, the voidspace occupied by said particle being substantially less than the volumeof said void, with one generally cross-sectional dimension of saidparticle at least approximating a corresponding cross-sectionaldimension of said void; the population of voids in said substrate andthe thickness of said substrate being such as to cause a degree ofopacity of less than 70 percent light transmission; and

(ii) a void-free thermoplastic skin layer adhering to the upper surfaceof said substrate layer, said skin layer being of a thickness such thatthe outer surface thereof substantially conceals the surfaceirregularities of said substrate layer; and,

(b) a release layer in the form of a thermoplastic film having an uppersurface and a lower surface and having a release agent component for thepressure sensitive adhesive component of label layer (a)(i) incorporatedtherein or applied as a coating to the upper surface thereof, the lowersurface of substrate (a)(i) being peelably affixed to the upper surfaceof said release layer (b).

In the simplest form, core layer (a)(i) which has been compounded with apressure sensitive adhesive component is coextruded with skin layer(a)(ii) and release layer (b) which has been compounded with a releaseagent to provide an integral label stock product. In addition toproviding a product having an aesthetically pleasing lustrous, opaqueappearance, the manufacturing operations involved herein are relativelysimple and few in number compared to the conventional manufacturingprocedure described above.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to achieve the lustrous satin appearance of the laminateconstituting facing layer (a) of the present invention, it is importantthat a particular thickness relationship exist between the thicknessdimension of substrate layer (a)(i) and the thickness dimension of skinlayer (a)(ii). It is preferred that the thickness be from about 30 toabout 85% of the overall structure. This, in combination with thepopulation and configuration of the voids in a structure of at leastabout 0.5 mils thick, will materially contribute to the overall degreeof opacity of the structure. Likewise, by maintaining the thickness ofskin layer (a)(ii) within a particular range in relation to the overallthickness of facing layer (a) and to the thickness of substrate layer(a)(i), the overall combination will exhibit a lustrous satinappearance. It is preferred that the thickness of skin layer (a)(ii)constitute from about 15 to about 70% of the total thickness of facinglayer (a). It is important that skin layer (a)(ii) be sufficiently thickso that its outer surface will not manifest the rough texture or surfaceirregularities of underlying substrate layer (a)(i). If this were notthe case, the lustrous satin-like appearance of skin layer (a) would besignificantly lessened.

It is preferred that the average diameter of the void-initiatingparticles contained within substrate layer (a)(i) be from about 0.1 toabout 10 microns. These particles should be approximately spherical inshape so as to initiate the formation of voids of unusual regularity andorientation in a stratified relationship throughout the matrix materialfollowing biaxial orientation of the entire coextruded product. Thisdoes not mean that every void must be the same in size. It means,generally speaking, that the voids will be of a similar shape eventhough they may vary in their dimensions because they are all initiatedby a substantially spherical particle. Ideally, all of the voids possessa shape defined by two opposed and edge-contacting concave disks.

Optimum characteristics of opacity and satin-like appearance areobtained when the two average major void dimensions are greater thanabout 30 microns.

As indicated above, the void-initiating particle material should beincompatible i.e., immiscible, with the matrix polymer of substratelayer (a)(i) at least at the temperature of biaxial orientation.

Substrate layer (a)(i) has been described above as a thermoplasticpolymer matrix material within which is located a strata of voids. Fromthis it is to be understood that the voids create the matrixconfiguration. The term "strata" is intended to convey the understandingthat there are a large number of voids creating the matrix and the voidsthemselves are oriented so that the two major dimensions are aligned incorrespondence with the direction of orientation of the polymeric filmstructure. After each void has been formed, void-initiating particlesmay contribute little else to the substrate layer. This is because itsrefractive index can be close enough to that of the matrix material thatit makes no contribution to opacity. When this is the case, opacity isprincipally a result of the light scattering effect brought about by theexistence of the voids. The opacity of the substrate layer can beincreased somewhat by incorporating an opacifying pigment therein.Titanium dioxide and zinc oxide are representative of such opacifyingpigments. A particularly preferred proportion of pigment in substratelayer can be from about 1% to about 3% by weight of this layer. Theparticle size and shape of the pigment material is such that it does notbring about any significant amount of void formation. The optionalpresence of such an opacifying pigment contributes perhaps 3 to 8% tothe degree of opacity of the system.

A typical void in substrate layer (a)(i) may be defined as having majordimensions X and Y and minor dimension Z where dimension X is alignedwith the machine direction orientation, dimension Y is aligned with thetransverse direction orientation and dimension X approximatelycorresponds to the cross-sectional dimension of the spherical particlewhich initiated the void. It is a necessary part of the presentinvention that orientation conditions be such that the X and Ydimensions of the voids present in the substrate layer be majordimensions in comparison to the Z dimension. Thus, while the Z dimensiongenerally approximates the cross-sectional dimension of the sphericalparticle initiating the void, the X and Y dimensions must besignificantly greater. In addition, the orientation conditions must besuch that the general integrity of the voids is maintained. By this, itis meant that during the orientation which produces the X and Ydimensions, that is, either by simultaneous or sequential machinedirection and transverse direction stretching, the temperatureconditions must be such as to permit these major dimensions to formwithout any destruction of the voids in any of their dimensions. Thevoids are particularly vulnerable to destruction during sequentialorientation if the stretching temperature is too low. Even insimultaneous orientation, if the temperature is too low the stretchingforces will tend to cause internal shredding and void splitting. Thisleads to a complete loss of control over the integrity of the individualclosed voids and as a consequence, a deterioration in the integrity ofthe matrix polymer. Thus, one skilled in the art following the presentgeneral guidelines can orient at a temperature and to a degree whichwill yield Z and Y dimensions approaching a maximum without causing anysubstantial amount of splitting, shredding or overall lack of void andmatrix integrity.

By way of illustration, room temperature biaxial orientation of apolypropylene matrix containing nylon spheres of the size and amountcontemplated herein will not produce the structure constituting facinglayer (a) herein. Either void splitting will occur or voids ofinsignificant size will result. Polypropylene must be oriented at atemperature which happens to be significantly higher than its glasstransition temperature. The temperature conditions must permit X and Yto be at least several multiples of the Z dimension without causing anyappreciable amount of void splitting. When this is accomplished, optimumphysical characteristics including low water vapor transmission ratesand a high degree of light scattering are obtained without significantvoid splitting or film fibrillating.

As indicated above, the matrix polymer and the void initiating particlecontained therein must be mutually incompatible, which is to say, theymust maintain two distinct phases under the conditions of biaxialorientation selected. The spherical void-initiating particles constitutea dispersed phase within the lower melting matrix polymer which polymerwill, upon orientation, become a void-filled matrix with the sphericalparticles positioned somewhere within the voids.

Prior to extrusion of substrate layer (a)(i), the polymer matrixconstituting the bulk of this layer is preferably admixed with asuitable quantity of pressure sensitive adhesive composition, e.g., fromabout 5 to about 30 weight percent, and preferably from about 10 to 20weight percent, thereof so that upon subsequent biaxial orientation ofthe assembled substrate layer (a)(i), skin layer (a)(ii) and releaselayer (b), the pressure sensitive composition, generally being amorphousin nature, will tend to disproportionately concentrate at each surfaceof substrate layer (a)(i). Such disproportionate concentration isthought to come about as a result of the orientation process whichbrings about crystallization of the matrix polymer accompanied by anexclusion of amorphous material (i.e., the pressure sensitive adhesivecomposition) from the crystalline areas of the matrix polymer, therebeing a tendency for such amorphous material to be driven or forced tothe surfaces of the substrate layer. The pressure sensitive adhesivecomponent generally comprises a normally non-tacky elastomer, e.g.,natural and/or synthetic rubbers, butadiene-styrene rubbers,polyisobutylene rubbers, isopolybutadiene rubbers, ethylene-propyleneterpolymer rubbers, polyacrylic rubbers, polyvinyl ether rubbers,butadiene-acrylonitrile rubbers, butadiene-styrene-acrylonitrilerubbers, butadiene-styrene-acrylonitrile rubbers, butyl rubbers, etc.,in combination with a tacky or tackifying resin, e.g., a rosin ester, ahydrogenated rosin ester, the glycerol ester of hydrogenated rosin, apolyterpene resin, a coumaroneindene resin, crude rubber, apolyisobutylene or other unsaturated hydrocarbon resin, a phenolatedterpene, a petroleum resin, etc. The pressure sensitive adhesive mayalso contain additives such as anti-oxidants, fillers, and the like.

Alternative to incorporating the pressure sensitive adhesive intosubstrate layer (a)(i), said layer together with skin layer (a)(ii) canbe coextruded followed by application of an organic solvent solution ofthe adhesive, e.g., in toluene, to the exposed surface of substratelayer (a)(i) in a separate operation with subsequent drying. However,this procedure results in additional manufacturing steps and as such isgenerally less desirable than the procedure of incorporating theadhesive into the polymer matrix prior to extrusion as described above.

Similarly, it is preferred to incorporate the release agent into thepolymer constituting release layer (b) prior to extrusion as it, too,will tend to concentrate at the surfaces of this layer following biaxialorientation. The amounts of release agent used can be within the rangesmentioned for the pressure sensitive adhesive. Like the latter, therelease agent can also be applied to the surface of release layer (b) ina separate operation. The release agent can be selected from any of thematerials known to have been used for their release properties forpressure-sensitive adhesives. Common types are silicones and modifiedsilicones, the modification including both copolymerization or siliconeswith other non-release chemical agents or by adding non-siliconematerials to a silicone coating solution prior to surface application ofthe release agent to the release layer. Other release agents such aspolyethylene, fluorocarbon, the Werner type chromium complexes andpolyvinyl octadecyl carbamate have also been used. The choice of therelease coating is dependent on the tack and adhesion level of theadhesive contained in the adjacent substrate layer as is appreciated bythose skilled in the art.

As a result of the biaxial orientation of combined facing layer (a) andrelease layer (b), in addition to opacifying substrate layer (a)(i) ofthe facing layer, the orientation improves other physical properties ofthe composite layers such as flex-cracking resistance, Elmendorff tearstrength, elongation, tensile strength, impact strength and coldstrength properties. In addition to this, a lustrous satin appearance isimparted to the film structure. As indicated above, this appearance isnot exhibited in the absence of the skin layer nor if the skin layer istoo thin or otherwise substantially reveals the surface imperfections ofthe substrate layer. If desired, a second skin layer (c), preferablyidentical in composition and thickness to skin layer (a) (ii), can becoextruded or separately affixed to the underside, or exposed surface,of release layer (b).

It is believed that because of the comparative sphericity of thevoid-initiating particles, the voids are closed cells. This means thatthere is virtually no path open from one side of the substrate to theother through which liquid or gas can traverse.

The void-initiating particles can be organic or inorganic so long asthey are spherical in shape and within the preferred particle size rangeso as to initiate the formation of a void, which in a significant numberof cases, has a lens-like shape, that is, a lens of the biconvex shape.Even a polymeric material can be used as the void-initiating particleprovided it is one which can be co-melted with the polymer of the matrixor substrate. In this case, it is necessary for it to have asufficiently higher melting point than the substrate polymer and beincompatible and capable of assuming a dispersed phase of smallspherical particles as the temperature of the co-melted mix is reduced.It is also contemplated that the void-initiating particles can bepreformed and then uniformly dispersed into a melt, e.g., one ofpolypropylene. This has the advantage of not subjecting the matrixpolymer to the temperature of a much higher melting polymer. In thismanner, any thermal degradation of the matrix polymer is avoided.

It is believed that because of the number, shape and orientationstrata-wise of the voids, a significantly enhanced light scatteringeffect is obtained. This effect is further enhanced or magnified by theuse of the skin layer of the size relationship mentioned above. Skinlayer (a)(ii) can optionally contain an opacifying pigment, e.g.,titanium dioxide to further augment the degree of opacity of facinglayer (a).

When preformed spheres are employed, it is the shape and size of thespheres that are important to the practice of this invention rather thantheir chemical composition. Solid or hollow organic or inorganic spheresof any type can be employed. Interesting effects can be achieved by theuse of spheres of different colors. Since statistically each void hasapproximately one sphere somewhere within the void, interesting andpleasing color and/or reflectance effects can be imparted to the overallfacing layer structure by the use of spheres of different colorabsorption or reflectance characteristics. The light scattered in aparticular void is additionally either absorbed or reflected by thevoid-initiating sphere and a separate color contribution is made to thelight scattering in each void.

Thermoplastic resins which can be employed as the dispersed phase withinthe matrix material include the polyamides or nylons of commerce,polyesters such as the polyethylene terephthalate acetals, acrylicresins, etc. Inorganic materials include solid or hollow preformed glassspheres, metal beads or spheres, ceramic spheres, etc. In fact, anymaterial which can be formed into spheres but does not cause thermaldegradation of the matrix material is within the scope of thisinvention.

By the technique of the present invention, light transmission throughthe facing layer (a) can be reduced to as low as about 16 percent. Thiswould be the case with a film having an overall thickness of at least1.5 mils where the thickness of substrate layer (a)(i) of the overallproduct is at least 60 percent with the thickness of skin layer (a)(ii)and release layer (b) each contributing about 20 percent to the overallthickness.

While the preferred particle size of the sphere is to be from about 0.1to about 10 microns, it is particularly preferred that the particle sizerange from about 0.75 to about 2 microns. The void-initiating particlescan be present in up to about 20 percent by weight of the core layerprior to orientation, a preferred range being from about 2 to 7 percentby weight.

For convenience and more precise control of the formulation andcharacter of substrate layer (a)(i), a master batch technique can beemployed either in the case of forming the spherical particles in situor in adding preformed spheres to the molten substrate matrix material.After the formation of a master batch, appropriate dilution of the batchcan be accomplished by adding additional thermoplastic substrate matrixmaterial until the desired proportions are obtained.

As previously stated, it is preferred that substrate layer (a)(i), skinlayer (a)(ii) and release layer (b) be coextruded. Thereafter, biaxialorientation is carried out to an extent and at a temperature calculatedto obtain the maximum degree of opacity without any significantsacrifice in the physical characteristics appearance of the filmstructure. As is evident, as the materials employed change, theconditions of biaxial orientation will change. By way ofexemplification, when nylon-6 is used as the void-initiating spheres, amachine direction orientation of from 4 to 8 times and a transversedirection orientation of from about 4 to 8 times at a drawingtemperature of from 100° C. to 160° C. yields a biaxially orientedpressure sensitive label stock product of from 0.7 to 3 mils overallthickness.

The following examples are illustrative of the invention.

EXAMPLE 1

A mixture of isotactic polypropylene (83 parts, MP 160° C. and a meltindex of 4.5), nylon-6 (7 parts, MP 225° C.) and a 1:2 weight partmixture of styrene-butadiene rubber and hydrogenated rosin derivative(10 parts) as a pressure sensitive adhesive is melted in a firstextruder provided with a screw of L/D ratio of 20:1 to provide thesubstrate layer. A second extruder in association with the firstextruder is supplied with the same polypropylene but without the nylon-6present to provide the skin layer and a third extruder in associationwith the first two extruders is supplied with the same polypropylene asthe second extruder but containing a standard silicone release agent (10parts) to provide the release layer. A melt coextrusion is carried outwhile maintaining the cylinder of the substrate material at atemperature ranging from 190° C. to 220° C. The polypropylene to beextruded as the skin layer and the release layer is maintained at atemperature of 220° C. A film structure is coextruded with a substratethickness of 40% of the total extruded thickness. The skin layer andrelease layer each is approximately 30% of the total thickness. Theunoriented film structure measures approximately 40 mils in thickness.This film structure is subsequently oriented seven by seven and one-halftimes using a commercially available sequential biaxial orientationapparatus causing void formation and opacification, solid statecrystallization of the polypropylene and forcing the pressure sensitiveadhesive and release agent to disproportionately concentrate at thesurfaces of their respective layers. The MD orientation temperature isabout 105° C. and the TD orientation 135° C. The resulting 1.9 milcoextruded pressure sensitive label stock has an opacity of 20%transmission and a 45° gloss of 120%. The combined substrate layer andskin layer, i.e., the facing layer, is readily peelable from the releaselayer and shows good surface adhesion.

EXAMPLE 2

Example 1 is substantially repeated except that a 35 weight percentsolution of the pressure sensitive adhesive in toluene is coated uponthe exposed surface of the substrate layer which has been coextrudedwith the skin layer. Following drying of the adhesive, the substratelayer/skin layer, i.e., the facing layer, is adhered to the separatelyextruded release layer and the combined layers are biaxially oriented asbefore to provide a pressure sensitive label stock product.

EXAMPLE 3

Example 2 is substantially repeated except that the silicone releaseagent is applied as a coating upon the separately extruded releaselayer.

EXAMPLES 4-5

Examples 1 and 2 are substantially repeated except that a second skinlayer identical to the first is applied to the underside, or exposed,surface of the release layer.

What is claimed is:
 1. An opaque, biaxially oriented label stock productof lustrous appearance which comprises:(a) a facing layer which is alaminate of(i) a thermoplastic polymer matrix substrate layer having anupper surface and a lower surface, a pressure sensitive adhesivecomponent incorporated within said substrate layer or applied as acoating to the lower surface thereof and a strata of voids locatedwithin said substrate layer; positioned at least substantially within atleast a substantial number of said voids is at least one sphericalvoid-initiating particle which is phase distinct and incompatible withthe matrix material, the void space occupied by said particle beingsubstantially less than the volume of said void, with one generallycross-sectional dimension of said particle at least approximating acorresponding cross-sectional dimension of said void; the population ofvoids in said substrate and the thickness of said substrate being suchas to cause a degree of opacity of less than 70 percent lighttransmission; and (ii) a void-free thermoplastic skin layer adhering tothe upper surface of said substrate layer, said skin layer being of athickness such that the outer surface thereof substantially conceals thesurface irregularities of said substrate layer; and, (b) a release layerin the form of a thermoplastic film having an upper surface and a lowersurface and having a release agent component for the pressure sensitiveadhesive component of label layer (a)(i) incorporated therein or appliedas a coating to the upper surface thereof, the lower surface ofsubstrate (a)(i) being peelably affixed to the upper surface of saidrelease layer (b).
 2. The label stock product of claim 1 wherein thevoid-initiating particle is an inorganic material.
 3. The label stockproduct of claim 1 wherein the void-initiating particle is an organicmaterial.
 4. The label stock product of claim 1 wherein thevoid-initiating particle is a polymer.
 5. The label stock product ofclaim 1 wherein a skin layer (c) is affixed to the exposed surface ofrelease layer (b).
 6. The label stock product of claim 1 wherein saidproduct is a coextruded structure.
 7. The label stock product of claim 5wherein said product is a coextruded structure.
 8. The label stockproduct of claim 1 wherein substrate layer (a)(i) and skin layer (a)(ii)are fabricated from the same resin.
 9. The label stock product of claim8 wherein the resin is isotactic polypropylene.
 10. The label stockproduct of claim 1 wherein substrate layer (a)(i), skin layer (a)(ii)and release layer (b) are fabricated from the same resin.
 11. The labelstock product of claim 5 wherein substrate layer (a)(i), skin layer(a)(ii), release layer (b) and skin layer (c) are fabricated from thesame resin.
 12. The label stock product of claim 10 wherein the resin ispolypropylene.
 13. The label stock product of claim 11 wherein the resinis polypropylene.
 14. The label stock product of claim 1 wherein thepressure sensitive adhesive is disproportionately concentrated at thesurfaces of substrate layer (a)(i).
 15. The label stock product of claim1 wherein the release agent is disproportionately concentrated at thesurfaces of release layer (b).
 16. The label stock product of claim 5wherein the pressure sensitive adhesive is disproportionatelyconcentrated at the surface of substrate layer (a)(i) and the releaseagent is disproportionately concentrated at the surface of release layer(b).
 17. The label stock product of claim 1 wherein the pressuresensitive adhesive is a combination of a natural or synthetic rubber anda tacky or tackifying resin and the release agent is a silicone ormodified silicone.